The present invention claims priorities from Japanese Patent Applications No.9-205706 filed Jul. 31, 1997, No. 9-210562 filed Aug. 5, 1997 and No. 9-273268 filed Sep. 22, 1997, which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an Optical Add-Drop Multiplexer (OADM) capable of switching a transmission line of a wavelength-multiplexed optical signal without electrically terminating the optical signal and an optical signal monitor utilizing the OADM.
2. Description of the Related Art
As the optical network, a network constructed on the basis of SONET (Synchronous Optical Network)/SDH (Synchronous Digital Hierarchy) using the time division multiplexing (TDM) has been used popularly. In a network based on this SONET/SDH, the switching of transmission line is usually performed by terminating an optical signal by a line layer, switching the transmission line and then multiplexing the optical signal again.
In this transmission line switching, in order to maintain the reliability of link connection, reliability evaluation values such as loss of frame (LOF), loss of pointer (LOP), bit error rate (BER), etc., are detected by detecting loss of signal (LOS) as described in, for example, GR-253-CORE (Issue 1, December 1994) describing Bellcore (Bell Communications Research) standard or evaluating section overhead (SOH) in a frame or line overhead (LOH).
Recently, an OADM which is capable of performing the transmission line switching without electrically terminating an optical signal has been proposed and its introduction into a practical system has been studied. Contrary to the transmission line switching in the digital cross-connect system (DCS) which is one of network elements of the SONET is performed through opto-electric (O/E) conversion, a demultiplexer (DEMUX), a switching, a multiplexer (MUX) and an electro-optical (E/O) conversion, the transmission line switching in the OADM can be done by only the switching and the DEMUX and the MUX for the electric signal are unnecessary. Therefore, it is possible in the OADM to reduce amount of hardware per transmission optical signal rate and to reduce the cost and the system size.
In a case where some failure occurs in a network having an OADM, the OADM is required to detect and judge faults such as loss of signal light due to breakage of optical fiber and degradation of signal light quality due to failure of such as optical repeater amplifier and to perform a recovering operation corresponding to the kind of fault In a case where a network fault is detected by utilizing a standard frame overhead in the currently used SONET/SDH, there is a necessity occurs in the OADM of terminating a section or line.
When a fault occurs in the line including the OADM, in order to notify an end user of the line the occurrence of the fault, read/write processing of an alarm indication signal (AIS) from the frame overhead becomes necessary in the OADM. In an OADM dealing with a high density multiplexed optical signal, however, there are problems that it is necessary to not only provide a large scale hardware in order to obtain the AIS from the frame overhead of the transmitted optical signal, but also electrically terminate the multiplexed optical signal, read the frame overhead and multiplex the optical signals again in order to merely read a fault monitor information within the network.
Further, in order to introduce the OADM into the currently existing optical network, it is necessary to deal with the optical signal separately from the time-multiplex orientated SONET/SDH layer, electrically process it synchronously in time and define a wavelength/space multiplexing orientated layer which does not obtain an information from that optical signal or does not add information thereto as an optical layer to thereby make an automatic protection system (APS) which is network fault recovery means supplied by the SONET/SDH non conflict with a protection system in the optical layer newly supplied by the OADM.
This is because, even if the SONET/SDH be disappeared in an ideal construction of future optical network, the resources of the SONET/SDH must be used for at least the present since the SONET/SDH is currently in world wide use and, since it is difficult in view of cost to immediately change from the APS to the protection system by the OADM, it is necessary to exist APS together the protection in the optical layer using the OADM for at least the present.
Considering the introduction of the OADM into a practical network, the ability of the OADM can not be fully used regardless of the high potential thereof which makes the reduction amount of hardware per transmitted optical signal possible, due to the above mentioned problems. In order to use the ability of the OADM sufficiently, it is indispensable to develop a system for monitoring a network fault and recovering the fault suitably for a network dealing with high density multiplexed signal and a hardware therefor while maintaining the compatibility with respect to the SONET/SDH.